Apical cells of 5-day-old dark-grown protonemata of the moss Ceratodon purpureus (Hedw.) Brid. are negatively gravitropic and appear to utilize amyloplasts as statoliths. These cells exhibit a characteristic plastid zonation (five zones) with one zone (No. 3) specialized for the lateral sedimentation of amyloplasts. Basipetal centrifugation displaces all amyloplasts in the apical cell to the end wall. In basipetally centrifuged protonemata observed using infrared videomicroscopy, tip extension occurred with or without amyloplasts present in the apical dome. The initial return of upward curvature was always correlated with the return and sedimentation of amyloplasts in zone 3. Subsequent vigorous upward curvature was correlated with distinct amyloplast zonation and further sedimentation in zone 3. Initial downward ("wrong way") curvature, which often preceded upward curvature, correlated with the presence of amyloplasts in the apical dome (zone 1). These data support the hypotheses that nonsedimenting amyloplasts in zone 1 are necessary for initial downward curvature and that amyloplast sedimentation in zone 3 is necessary for upward curvature.